The roundness measuring apparatus which measures a roundness of a cylinder outer surface or a cylinder inner surface of an object (work) to be measured is used widely.
FIG. 1 is an outline view of a conventional roundness measuring apparatus.
The roundness measuring apparatus includes: a table-like base 1; a rotatable stage 2 provided at the base 1; a rotation drive part 3 having a motor, etc., for rotating the stage 2; a column 4 provided at the base 1; a carriage 5 which is movable along the column 4; an arm 6 which is movable with respect to the carriage 5; a detector holder 9 attached to a tip of the arm 6; and a detector 10 attached to the detector holder 9. The detector 10 includes a probe 11 and a displacement detection part such as a differential transformer, and outputs an electrical signal which shows displacement of the probe 11.
A work 12 is placed on the stage 2 so that a center axis of a cylinder surface of the work 12 almost coincides with a rotation axis of the stage 2, and the work 12 is rotated. The column 4 is a pillar extending in parallel to the rotation axis of the stage 2. The carriage 5 is movable along the column 4. Generally, the carriage 5 is manually moved along a guide surface of the column 4, but it may be moved automatically using a motor, etc. The arm 6 is manually moved along a guide surface of the carriage 5, but is may be moved automatically using a motor, etc. The detector holder 9 is a component having an L shape, one end of the detector holder 9 is attached at the tip of the arm 6, and the detector 10 is attached to the other end of the detector holder 9. In order to detect a change of height position of a flange portion of the work 12, the detector holder 9 may be able to be attached at the tip of the arm 6 with three different directions by a unit of 90 degrees. Further, in order to change a direction of the cylinder surface to be measured by 180 degrees, the detector 10 may be able to be attached to the detector holder 9 at different directions.
When the measurement is carried out, the work 12 is placed on the stage 2 so that the center axis of the cylinder surface of the work 12 almost coincides with the rotation axis of the stage 2. In order to contact the probe 11 at a position of the work 12 to be measured, a vertical position of the probe is adjusted by moving the carriage 5 and a position of radial direction is adjusted by moving the arm 6. A roundness of the work 12 is measured in this state. When a high-precision measurement is carried out, eccentricity of the center axis of the cylinder part of the work 12 from the rotation axis of the stage 2 is measured by rotating the work 12, and the rotation axis of the stage 2 is adjusted to correctly coincide with the rotation axis of the stage 2 by using an XY moving mechanism provided at the stage 2. At this time, the probe 11 is desirable to be near a center of a displacement range.
FIG. 2 is a plan view of the roundness measuring apparatus of FIG. 1.
As illustrated in FIG. 2, the column 4 is provided at the right-hand side of the stage 2. The arm 6, the detector holder 9 and the detector 10 are arranged on a straight line, and the rotation center axis of the stage 2 is placed on extension of the straight line. The probe 11 is provided at a tip of the detector 10, and the probe 11 is displaced on a plane formed by this straight line and the rotation axis of the stage 2. Therefore, when the work 12 having a cylinder surface of a different diameter is measured, the arm 6 is moved so that the probe 11 contacts the cylinder surface to be measured. In this description, a plane formed by a measurement point on a cylinder surface to be measured at which the probe 11 contacts and the rotation center axis of the stage 2 is called a measurement plane, and a direction connecting the rotation center axis of the stage 2 and the measurement point is called a radial direction. In other words, even when cylinder surfaces having different diameters are measured, the arm 6, the detector holder 9 and the detector 10 are moved in the radial direction along the measurement plane, and the probe 11 contacts with a cylinder surface to be measured on a line at which the measurement plane and the cylinder surface cross, and displaces on the measurement plane.
A first reason for providing the column 4 on the right-hand side of the stage 2 (left-hand side may be also acceptable) is for moving the arm 6 on a measurement plane. A second Reason is for enabling to detect a difference of radiuses (diameters) of cylinder surfaces of different cylinder surfaces is detectable by detecting movement magnitude of the arm 6.
For the above reasons, the column 4 has been provided at one side (right-hand side or left-hand side) of the stage 2 in conventional roundness measuring apparatuses. Therefore, the base 1 where the column 4 is fixed has a rectangle shape being long in a radial direction in the plan view. Since the arm 6 is moved in the radial direction according to radiuses of cylinder surfaces to be measured, a setting space needs to be determined in consideration of the case where the arm 6 moves to the utmost rightward. For the above reasons, the conventional roundness measuring apparatuses need the setting space of a rectangle shape being long in the radial direction, and there has been a problem that a space required for installation is large.
Since a conventional roundness measuring apparatus has a long arm 6, there has been a problem that a displacement of the detector 10 attached at the tip of the arm 6 and the detector holder 9 is large even if there is a small temperature change in a short time during measurement. Further, since the displacement directly influences a measurement value, there has been a problem that an error due to the temperature change is large.